Numerous studies have examined the relationship between serum ferritin concentrations and iron status. These investigations have been done primarily with adults and reveal that serum ferritin is a good indicator of uncomplicated iron deficiency. The current techniques for the determination of serum ferritin levels involve the utilization of two-site immunoradiometric assays, available only to those laboratories that are licensed to do RIA procedures. We propose to develop and validate an enzyme immunoassay which will employ a serum-cholinesterase - ferritin antibody conjugate in place of the radioactively labeled antibody such as used in RIA. This type of enzyme immunoassay will provide a more available assessment for serum ferritin concentration than previously utilized methodology. Validation for this enzyme immunoassay will be done by comparison with an immunoradiometric assay being established. The enzyme immunoassay will be used to determine the serum ferritin concentrations of over 2,400 samples that will be obtained by an on-going (U.S.-D.H.E.W.) research grant "Iron Nutrition and Growth of Infants". The dietary information, growth data, and biochemical indices derived from this study will be examined relative to serum ferritin concentrations to determine the relationship, if any, between serum ferritin and infant iron nutrition during the first two years of life.